1. Field of the Invention
The present invention relates to an endoprosthesis to apply in bone surgery, without using surgical cement, and more particularly to a prosthetic acetabulum and head.
2. Brief Description of the Related Art
There are known endoprosthesis the connection of which with the bone takes place by the growing into pores of proper dimensions, into indentations and other irregularities on the surface of the endoprosthesis, fixed on the spot where it is mounted with the help of surgical cement, or mechanically. In order to stop bone loss, it has been proposed to use an endoprosthesis joined with the bone by pushing surgical cement into the marrow cavity for example, polymethylmethacrylate cement, or with the help of coarse threads. These endoprostheses suffer from a very small bonding surface formed by ingrowth of spongy bone.
Furthermore, the joining of an endoprosthesis to the bone by surgical cement has some defects. The main defect is that the material weakens as it ages, which causes it to fail. Consequently, irritation from the failing cement causes a separation from the bone and this, in turn, causes loosening of the endoprosthesis and the bone's atrophy.
DE 3443109 A1 describes a prosthesis in which the prosthetic implants have more secure fastenings by applying conic irregularities which stick out of the terminal surface of the endoprosthesis and which are directed against the 'joining surface of the bone. The conic irregularities are pressed into the spongy bone and cause an increase in the contact surface which is favourable to fixation of the prosthesis to the bone by bone ingrowth. An imperfection of this solution is the fact that there is no possibility to optimally increase the contact surface of the endoprosthesis with the bone because the conic irregularities are spaced apart on a plane surface. However, the plane contact surface, because of it's shape, renders impossible ingrowth of spongy bone in a shape similar to the natural form, which would assure a good absorption of the joint's forces.
FR A 2 519 545 describes a prosthesis in which the acetabulum has a total spherical form and also, on the top of the endoprosthesis, includes conic irregularities which stick out of the boundary surface of the endoprosthesis and which are directed against the joining surface of the pelvic bone; this structure, however, is not adapted to the physiological structure of the bone. For this reason it can not be implanted due to the fact that the "bottom" of the acetabulum in the bone of the pelvis is flat, hard and made out of a thin cortex in the pelvic bone. Therefore, the needle on the top of the acetabulum cap can cause a perforation of the bone, which causes a weakening and finally a damage thereto: a bleeding of the blood vessels inside the pelvis.
Another deficiency of the device described in FR A 2 519 545 is the insertion of the endoprosthesis, which occurs at the same time as implantation of the needles in the cap. Furthermore, insertion of the endoprosthesis in such a form, in which all of the needles have the same length and extend over the highest point of the external surface of the acetabulum, is imperfect because of the anatomical structure of the pelvis bone. Such an installation of the endoprosthesis makes impossible bone ingrowth between the sharply ended needles, and therefore causes a weak connection of the bone with the endoprosthesis, because the connection is purely mechanical and causes unfavourable transmission of the biomechanical forces of the hip joints.
FR A 2 686 503 describes a prosthesis in which the endoprosthesis facilitates only a partial reconstruction of the femoral head surface, which has been removed because of necrosis. The imperfection of this invention is that the endoprosthesis does not permit resection of the whole femoral head surface. Additionally, implantation of the endoprosthesis requires a special hole for the keel 5, which complicates the surgery and diminished the adhesion. Because the endoprosthesis according to this solution diminished the adhesive interface surface, it is not stable for a non-cemented solution.